1. Field of the Invention
The invention concerns a process of making an ultraviolet (“UV”) curable pressure sensitive adhesives (“PSA”) by derivatizing an acrylic polymer, such as a hydroxy-functional acrylic polymer, with a derivatizing agent prepared from a diisocyanate and a hydroxy-functional acrylate monomer.
2. The Related Art
Solvent based PSA typically have low solids content, such as from about 40% to about 55% solids. The coating process from solution generally limits the effective coating thickness for common applications, as it is difficult to evaporate solvents from a thick film. However, for many applications, thicker films are desired. Hot melt adhesives offer an avenue to easily generate thick films. Historically, most hot melt adhesives are rubber-based, and have limitations in performance.
UV curable acrylic hot melt PSA offer a potentially attractive alternative to rubber based hot melt adhesives, as lightly crosslinked acrylic adhesives have premium peel and shear performance, as well as outstanding weathering and aging performance compared to rubber based adhesives. However, many attempts at a UV curable acrylic PSA have had limited commercial success for thick film applications. This is generally due to inefficiency in uniformly curing thicker films through the complete depth of the film.
A primary limitation for UV curable PSA in certain commercial applications is due to the nature of the photoinitiator entity. For these adhesive products, the photoinitiator is built into the acrylic polymer backbone by free radical polymerization of an acrylated benzophenone. The built-in benzophenone entity is incorporated at a level of about 0.5% to about 1.0%. The benzophenone group is a very strong UV absorber, and is a strong hydrogen abstracter when excited by UV. The hydrogen-abstraction process causes free radicals to be generated. When these free radicals recombine, it causes a slight crosslinking of the polymer. This crosslinking is the key to converting a melt-flowable polymer into an adequate PSA. However, there is a depth limitation for the cure of the benzophenone-based polymers. Due to benzophenone's strong absorbance at 258 nm, UV radiation at this wavelength cannot pass through a sample depth of greater than approximately 70 um (about 2.7 mils). Therefore, for practical purposes, a benzophenone based UV curable PSA can only be uniformly cured at thicknesses of 2 mils or less.
Other patents in the area of UV curable acrylic hot melt PSA include adhesive compositions comprising a combination of built-in photoinitiator and olefinic components to facilitate crosslinking. Separately, an acrylate functional polyurethane has been touted as UV curable warm melt PSA. All of these hot melt PSA are 100% solids with no volatile organic content (“VOC”) during application, eliminating the need for drying ovens and incinerators/thermal oxidizers.
All parts and percentages set forth herein are on a weight-by-weight basis unless otherwise specified.